1. Field of the Invention
The present invention relates to a waste liquid processing method and a waste liquid processing apparatus used in the method, for the purpose of selectively separating, decomposing by means of a photocatalyst, and thereby defusing a chlorine-based organic solvent contained in exhaust gas or waste liquid discharged from a process that uses the chlorine-based organic solvent such as trichloroethylene used in a cleaning apparatus for metal parts and tetrachloroethylene used in a dry cleaning apparatus.
2. Description of the Related Art
A chlorine-based organic solvent has been widely used as an excellent cleaning agent for its excellent cleaning performance, stability, and non-flammability.
Recently, since chlorine-based organic solvents were designated as priority procurement substance and voluntarily controlled substances by the Air Pollution Control Law because they are toxic materials that pollute the atmosphere, treatment methods such as adsorption onto active carbon and evaporation by heating have been employed to keep the concentration of the chlorine-based organic solvent contained in waste liquid and exhaust gas discharged from cleaning apparatuses, such as a dry cleaning apparatus, within permissible emission values (0.1 mg per liter for tetrachloroethylene) as specified in the Water Pollution Control Law.
FIG. 5 schematically shows an example of a prior art waste liquid processing apparatus.
In FIG. 5, reference numeral 1 denotes a solvent separation process section, and the arrows indicate the direction in which the chlorine-based organic solvent flows. The solvent separation process section 1 is connected to a vaporization process section 3 either directly or via a waste liquid supply line 2. Further connected to the vaporization process section 3 is a discharge line 9 that discharges effluent after aeration.
The solvent separation process section 1 is also connected, via a vaporization gas supply line 4, to a photo-oxidative decomposition process section 5, and the photo-oxidative decomposition process section 5 is connected to a post-treatment process section 7 via a decomposition product gas supply line 6. Further connected to the post-treatment process section 7 is a discharge line 10 that discharges non-toxic air containing post-treated water and carbon dioxide.
The waste liquid processing apparatus further comprises a control section 8 that controls the solvent separation process section 1, the vaporization process section 3, the photo-oxidative decomposition process section 5, and the post-treatment process section 7.
The solvent separation process section 1 selectively adsorbs the chlorine-based organic solvent that is dispersed in the form of minute particles in the waste liquid. Then the waste liquid containing the chlorine-based organic solvent that has not been adsorbed in the solvent separation process section 1 is aerated in the vaporization process section 3 so as to vaporize the chlorine-based organic solvent, and the vaporization gas containing the chlorine-based organic gas thus generated is passed through the solvent separation process section 1 again. When passing through the solvent separation process section 1, the vaporization gas vaporizes the chlorine-based organic solvent that has been adsorbed in the solvent separation process section 1, and the total combined vaporization gas is introduced into the photo-oxidative decomposition process section 5 through the combined vaporization gas supply line 4. Then the vaporization gas is decomposed in the photo-oxidative decomposition process section 5, and the decomposition product gas containing the chlorine-based gas generated by the photo-oxidative decomposition reaction is introduced into the post-treatment process section 7 through the decomposition product gas supply line 6, so as to be adsorbed, absorbed, and/or neutralized in the post-treatment process section 7 to be converted into non-toxic chlorides, thereby completing the treatment of waste water or exhaust gas.
The waste liquid discharged from a dry cleaning apparatus may include, in addition to the chlorine-based organic solvent, substances such as isopropyl alcohol, ethylene glycol, and surfactant. There has been a problem such that, when such substances are vaporized together with the chlorine-based organic solvent in the vaporization process section 3 and are introduced into the photo-oxidative decomposition process section 5 via the vaporization gas supply line 4, the capability of the photo-oxidative decomposition process section 5 to decompose the chlorine-based organic gas decreases, thus causing the chlorine-based organic gas to be released into the atmosphere without being decomposed. There is also a problem such that, as the capability of the photo-oxidative decomposition process section 5 to decompose the chlorine-based organic gas decreases, the efficiency of the treatment also decreases.